The invention is concerned with a single-rod pH measuring circuit comprising a pH-sensitive glass electrode and a reference electrode which is in electrically conductive communication with the solution to be measured via a salt bridge.
For measuring of the hydrogen ion concentration in a test solution there is normally used a measuring electrode, principally a glass electrode, in conjunction with a reference electrode, whereby both electrodes are immersed in the solution to be measured.
Such an electrode combination is called a pH measuring circuit. Thereby, the reference electrode must deliver a constant reference potential independently of the composition of the test solution. On the other hand, there eventuates a potential at the glass electrode which is in exact function with the hydrogen ion concentration or the pH-value of the solution.
The variation in potential of a pH measuring circuit is measured with amplifiers of very high input impedance and is somewhat temperature dependent. The variation in potential is 59.2 mV per pH unit at 25.degree. C.
The reference electrode normally consists of a vessel, principally of glass, filled with a suitable salt solution, preferably a concentrated aqueous solution of potassium chloride, sodium chloride or potassium nitrate. An electrode, usually calomel or silver-silver chloride, is immersed in this salt solution in order to derive the reference potential. The salt solution is in communication with the test solution via a suitable outflow aperture, hereinafter called the salt bridge. The outflow of the salt solution is normally attained by an elevated hydrostatic pressure as compared with the test solution. On the other hand, the too rapid outflow of salt solution must be prevented, wherefore the outflow aperture is usually formed from asbestos fibers, porous glass or ceramic rods, platinum strands, ground glass sleeves, one upon another.
The glass electrode consists of a glass tube which is closed at one end with a diaphragm consisting of electrically conductive special glass which is selectively sensitive to pH variations. The derivation of the diaphragm potential is effected via a buffer solution and a silver--silver chloride electrode.
On grounds of practical handling, the glass electrode and the reference electrode are often combined in a concentric structure. One then speaks of a single-rod pH measuring circuit or combination pH electrode.
The potentiometric pH measuring technique is one of the established analysis methods in applied and theoretical chemistry.
The pH influences the course of a large number of chemical reactions occurring in nature and industry, especially those of most biological-chemical processes.
In man the maintenance of the almost neutral reaction of the blood in the organism is a vital necessity, since a change in the acid-base equilibrium influences not only the ion antagonism and the respiratory function of the blood, but also completely determines in their properties the proteins in the blood, the cell membranes in the tissues and the enzymes.
In man, the pH range of the blood is fairly narrow-banded. In two test groups of twenty men and twenty women each, 95% of the pH values of capillary whole blood were only between 7.36 and 7.43.
If the pH value deviates from the mentioned range then there arise serious disturbances which can lead to brain damage and to death. Such phenomena occur relatively often in the foetus during birth and can cause permanent damage such as mental debility and paralysis.
As the extreme, pH values in the human organism are given pH 6.8 and 7.8.
The pH value is lowered (acidosis) in the case of oxygen deficiency, kidney failure and diabetic coma.
The pH value is increased (alkalosis) in the case of excessive breathing of oxygen in conjunction with anaesthesia or in the case of lasting acid loss, e.g. as a result of vomiting.
For years there has been used almost exclusively for the medical pH monitoring a discontinuous "in vitro method". It consists in blood-sampling by vein-puncture and subsequent "in vitro determination" of the pH value conventional apparatus.
In all cases, but especially in the case of those of obstetrics, there are present the following disadvantages:
1. Critical trends in the pH development are often only recognizable later.
2. An increased risk of infection results through many punctures.
3. The normal obstetric procedure is interfered with by the blood sampling.
In recent times there have been reported laboratory tests with special electrodes which are directly inserted into the blood vessels and which should permit the continuous pH measurement in living bodies ("in vivo").
However, continuous pH measurements in vivo in the case of foetuses have not hitherto been described and are regarded as technically impracticable, namely
1. because, during the birth, no sufficiently large vessel is present in the scalp which enables a safe introduction of such a special electrode, and
2. because, according to the present state of medical knowledge, a pH measurement outside the vessel gives no clinically usable result for the estimation of the foetal metabolic situation.
It has now surprisingly been found that the pH value in the interstitial space of the subcutis is the same as in the capillary blood and varies analogously as the changes in the pH value of the capillary blood. In the child's scalp, the interstitial space of the subcutis lies between the scalp and the periosteum. In the normal state, it embraces a gap of about 2 mm width which is filled with fine fibers, cell nuclei and tissue fluid.
The fact that, in the case of the foetus, pH measurements in the interstitial fluid of the subcutis give clinically equivalent results to intermittently undertaken pH analyses of capillary blood, is a completely unexpected finding having regard to previously accepted views in the relevant medical literature.
Most of the mentioned electrodes contain the measuring and reference electrodes built into an injection needle in order to protect these sensitive parts upon perforation of skin and tissue (see, e.g. U.S. Pat. Nos. 3,244,433; 3,244,436 and 3,415,731). Such an arrangement has a serious disadvantage: redox potentials, which occur at all metal surfaces, can influence the accuracy of the in vivo pH measurements. This source of error is not observable upon calibration of the electrodes, since the redox potentials remain constant in calibration buffer solutions. On the other hand, the redox potential of the blood can show fluctuations of 40 vM.
The redox potential of the blood may especially strongly influence pH measurements with an iridium needle (see e.g. U.S. Pat. No. 3,726,777).
However, the most important source of error in relation to the accuracy of the measurement is the unstable potential of the reference electrode relative to the test solution. In general, the coagulation system of the blood inactivates within a short time the salt bridge of the reference electrode by the deposition of solid protein layers. This is especially true for capillary tubes. There is thus the need for an insensitive salt bridge. For the stable formation of a reference potential it is necessary that a small amount of the reference solution flows out.
A further frequent source of error, especially with regard to the reliability of the measurement, consists in that the known constructions cannot ensure that the reference electrode is in electrically conductive contact with the solution to be measured via the salt bridge.
A dilution or contamination of the mentioned saturated reference solution can also occur by back diffusion. In the interest of a long service life it is necessary that the reference electrode can be opened, mechanically and chemically cleaned and further filled with fresh reference solution.